1. Field of the Invention
The present invention relates to an engine starting system for a power train which includes an engine and an automatic transmission for transmitting engine power while performing gear shifting operation in a prescribed manner, wherein the engine starting system is configured to automatically stop the engine when conditions for automatic engine stop are satisfied and to automatically restart the engine when conditions for engine restart are satisfied after automatic engine stop.
2. Description of the Related Art
Developments in recent years in automotive industry intended for reducing fuel consumption and carbon dioxide (CO2) emissions provide an engine starting system which automatically stops the engine at idle and automatically restarts the engine when engine restart conditions are satisfied as a result of a driver's action taken after idle stop for starting a vehicle, for instance.
Generally, engine restart requires promptness to instantly restart the vehicle when the engine restart conditions are satisfied. Therefore, a conventional method of engine restart in which the engine is restarted by cranking an output shaft of the engine by means of a starter motor is not so preferable because the conventional engine restart method requires a good deal of time to complete an engine start sequence.
Thus, it is desirable to supply fuel into a particular cylinder which was on an expansion stroke at engine stop and cause ignition and combustion to occur in that cylinder so that the engine can be instantly started with resultant combustion energy. For the convenience of explanation to follow in this Specification, the cylinder which was on the expansion stroke at engine stop is hereinafter referred to as the “expansion stroke cylinder.” Even if the fuel is supplied to and combusted in the expansion stroke cylinder, however, it will not always be possible to produce a sufficient torque for restarting the engine. For smooth engine restart, it is essential that the combustion in the cylinder produce a sufficient torque.
To ensure that the combustion in the expansion stroke cylinder produces a sufficient starting torque, a piston in the cylinder should preferably be located in an appropriate range at restarting. Generally, the appropriate range of stop position of the piston in the expansion stroke cylinder is a range at around 90 degrees after top dead center in terms of crank angle, that is, a range centered at a midpoint between the top dead center and bottom dead center or a range slightly offset therefrom. If the piston in the expansion stroke cylinder is stopped within this appropriate range at engine stop, air left in proper quantity in the cylinder and the fuel supplied thereinto would produce combustion in a desirable fashion, thus providing a sufficient torque for engine restart.
Aiming at a solution of the aforementioned problem, Japanese Unexamined Patent Publication No. 2004-124754 proposes an engine stop control technique for stopping a piston in a particular cylinder in an appropriate range. The technique described in this Publication is to regulate the amount of intake air during engine stop operation to provide a proper amount of in-cylinder air so that the piston in the cylinder can easily stop in the appropriate range. According to the Publication, an engine starting system employing this engine stop control technique can provide improved engine restartability as the piston in the particular cylinder can be controlled to stop in the appropriate range with high precision.
To perform the engine stop operation described in Japanese Unexamined Patent Publication No. 2004-124754 with yet higher precision, disturbances to a crankshaft of the engine should preferably be as low as possible during execution of the engine stop operation. One approach currently under study for reducing such disturbances is to disengage a starting clutch of an automatic transmission, or to shift the transmission from a drive condition to a neutral condition, when automatically stopping the engine.
The starting clutch disengagement approach however requires that the engine be instantly restarted and the automatic transmission be quickly shifted from the neutral condition back to the drive condition if there is made an engine restart request, such as depression of an accelerator pedal, when the automatic transmission is in the neutral condition during execution of the engine stop operation. To prevent a delay in this form of gear shifting operation, Japanese Unexamined Patent Publication No. 1999-351371 proposes an arrangement for setting a transmission in a precharged state during a process of automatic engine stop, the precharged state being a condition of the transmission in which a clutch is disengaged but a small amount of hydraulic pressure is supplied to the clutch to hold the same in a state immediately before engagement. This arrangement would make it possible to quickly engage the clutch in response to an engine restart request, such as depression of the accelerator pedal, thus ensuring quick restating and acceleration capabilities of the vehicle.
However, the aforementioned control operation (hereinafter referred to as precharge control operation) to maintain the automatic transmission in the neutral condition by precharging the clutch requires high-precision clutch engagement control. Holding the clutch in the precharged state is almost equal to, so to speak, keeping the clutch in balance at a boundary between engagement and disengagement. Keeping the clutch in such delicate balance is however not so easy in actuality, because engagement/disengagement operation of the clutch could be affected by variations in working fluid pressure and other factors. Additionally, if the clutch slightly deviates from an ideal precharged state to engagement side, the clutch will be brought into a loosely engaged state associated with slippage. Since the slippage would more or less lead to heat buildup in the clutch, the loosely engaged state may cause damage to the clutch. The precharge control operation of the aforementioned Publication requires high-precision clutch engagement control and poses a problem affecting reliability of the clutch as stated above and, thus, the precharge control operation is so difficult to carry out. Even if it is feasible to employ the precharge control operation of the clutch, implementation of the same in the automatic transmission would be possible only through engineering efforts undertaken over an enormously long period of time.